True RMS converter board

ABSTRACT

A true RMS conversion circuit. A test apparatus includes a transmitter source including AC and DC supplies. The transmitter source can connect to a transmitter. The test apparatus includes an optical receiver to receive optical signals from a transmitter coupled to the transmitter source. The optical receiver includes an amplifier with a differential output. The test apparatus also includes a matching networks connected to a high and low outputs. Also included in the test apparatus are filters connected to the matching networks. The filters are designed to separate AC and DC signals. An RMS conversion circuit is connected to the filters such that the RMS conversion circuit receives AC signals from the filters. The RMS conversion circuit converts the AC signals to a DC function of the RMS value of the AC signals. A data acquisition system is connected to the RMS conversion circuit to receive the function of the RMS value of the AC signals. The data acquisition system is connected to the filters to receive DC signals from the filters.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 60/599,259, titled “True RMS Converter Board” filed Aug. 4, 2004, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The invention generally relates to fiber-optic test equipment. More specifically, the invention relates to test equipment for testing optical transmitters and receivers used in fiber-optic communications.

2. Description of the Related Art

Fiber-optic networking can be used to communicate in modern high-speed networks. To transmit data on a fiber-optic network, the data must be converted from an electronic signal to an optical signal. This conversion may be done, for example, by using a transmitter or transmitting optical subassembly (TOSA). The transmitters and TOSAs often include light generating devices such as a laser or light emitting diode (LED). The light generating device is modulated according to digital data to produce a modulated optical signal.

When optical signals are received, those optical signals must generally be converted to an electronic signal. This is often accomplished using a receiver or a receiver optical subassembly (ROSA). Receivers and ROSAs generally include a photo sensitive device such as a photodiode connected to a transimpedance amplifier (TIA). When an optical signal impinges the photo sensitive device, a modulated current is induced in the photo sensitive device. This current can be converted by the TIA to an electronic signal usable by digital devices on a network.

Manufacturers of ROSAs and TOSAs typically perform various performance testing on the ROSAs and TOSAs before they are delivered to distributors and end customers. This performance testing can be used to detect defects or to sort components into groups of different rated values.

More particularly, testing directed towards the ROSA may include testing the responsivity of the ROSA to a modulated optical signal, testing the amount of current produced for a given amount of optical signal and so forth. Testing responsivity includes comparing a modulated optical signal input into the ROSA to an AC electrical signal produced by the ROSA as a result of receiving the AC optical signal.

Testing may be performed on the TOSA to characterize operating characteristics of the TOSA. One test that may be performed includes plotting the amount of optical energy produced by the TOSA as a function of the amount of current used to drive the TOSA. Another test includes measuring the amount of noise produced by the TOSA.

Many of these tests have conventionally been performed using expensive high-frequency test equipment. For example, some tests use a high frequency communications analyzer costing in the tens of thousands of dollars. Further, many of these test devices are general-purpose test devices. As such, these devices require excessive amounts of human interaction to perform the test result in and increase test times for each component. When each and every component manufactured is tested, this requires an inordinate amount of manpower and equipment to process testing of the components quickly.

Additionally, testing is often not repeatable from part to part. This is due to the changing nature of cables and the like associated with general purpose test equipment.

BRIEF SUMMARY OF THE INVENTION

One embodiment includes a test apparatus for testing transmitter devices such as laser diodes and LEDs. The test apparatus includes a transmitter source. The transmitter source includes an AC supply and a DC supply. The transmitter source is able to connect to a transmitter. The test apparatus further includes an optical receiver configured to receive optical signals from a transmitter coupled to the transmitter source. The optical receiver includes an amplifier with a differential output. The test apparatus also includes a first matching network connected to a high output of the differential output and a second matching network connected to a low output of the differential output. Also included in the test apparatus are a first filter connected to the first matching network where the first filter and a second filter connected to the second matching network. The first and second filters are designed to separate AC and DC signals. The first and second filters may be for example bias tees. An RMS conversion circuit is connected to the first and second filters such that the RMS conversion circuit receives AC signals from the first and second filters. The RMS conversion circuit converts the AC signals to a DC function of the RMS value of the AC signals. A data acquisition system is connected to the RMS conversion circuit to receive the function of the RMS value of the AC signals. The data acquisition system is connected to the first and second filters to receive DC signals from the first and second filters.

Another embodiment includes a test apparatus for testing receiver devices. The test apparatus includes a transmitter source. The transmitter source includes an AC supply and a DC supply. The transmitter source is connected to a transmitter. The test apparatus further includes a fixture configured to receive an optical receiver such that the optical receiver may receive optical signals from the transmitter connected to the transmitter source. The optical receiver may include an amplifier with a differential output. The test apparatus also includes a first matching network able to connect to a high output of the differential output and a second matching network configured to connect to a low output of the differential output. Also included in the test apparatus are a first filter connected to the first matching network where the first filter and a second filter connected to the second matching network. The first and second filters are designed to separate AC and DC signals. An RMS conversion circuit is connected to the first and second filters such that the RMS conversion circuit receives AC signals from the first and second filters. The RMS conversion circuit converts the AC signals to a DC function of the RMS value of the AC signals. A data acquisition system is connected to the RMS conversion circuit to receive the function of the RMS value of the AC signals. The data acquisition system is connected to the first and second filters to receive DC signals from the first and second filters.

Another embodiment includes a method of testing optical components. The method includes receiving an optical signal from an optical source. The optical signal including AC and DC components. The optical signal is converted to an electrical signal. The AC and DC components of the electrical signal are separated. The AC component of the electrical signal is converted to a function of the RMS value of the AC component of the electrical signal. The function of the RMS value is provided to a data acquisition system.

The embodiments described above allow for AC and DC testing to be completed on optical components using a single inexpensive test board. This can conserve resources as only a single test fixture is used to perform both sets of tests.

These and other advantages and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In order that the manner in which the above-recited and other advantages and features of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

FIG. 1 illustrates a block diagram overview of a test apparatus for testing optical components;

FIG. 2 illustrates a schematic drawing showing the general construction of a test apparatus for optical components; and

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, an embodiment of the invention is illustrated as a test apparatus 100 that provides for monitoring of AC and DC signals produced by a transmitter. The apparatus 100 shown in FIG. 1 may be used to test transmitter devices. In one example, the transmitter device may be a TOSA or a transmitter which includes a laser diode or light emitting diode (LED). The apparatus shown in FIG. 1 includes a detector device 102. The detector device 102 may be for example a detector that includes a photodiode coupled to a transimpedance amplifier with a differential output. The detector device 102 will be discussed in more detail with reference to FIG. 2 described herein. The detector device 102 outputs a positive output signal at a positive output signal terminal 104 and a negative output signal at a negative output terminal 106. The positive output signal is fed into a positive input port 108 of an RMS detector board 110. Similarly the negative output signal is input to a negative input port 112 of the RMS detector board 110.

The RMS detector board 110 includes circuitry for filtering DC components of the output signals from the AC components. The DC components of the signal are output at detector board DC output ports 114, 116. The DC output signals may be received by a data acquisition system 118 for use in characterizing the properties of a transmitter.

The AC portions of the signals are converted to a function of the RMS value of the AC signals which is then fed to a RMS output port 120. The function of the RMS value of the AC signal may be received by the data acquisition system 118 and used in characterizing various characteristics of the transmitter.

An alternate embodiment of the invention may be used to test receiver devices by using a transmitter fixed as a portion of the test apparatus 100. In this example, the detector device 102 may be removable such that testing can be performed on numerous detector devices or receivers produced by a manufacturer.

Referring now to FIG. 2, a circuit diagram illustrates various features of one embodiment. FIG. 2 illustrates a transmitter source 250. The transmitter source includes an AC supply 252 and a DC supply 254. The DC supply 254 may be used to bias a LED or laser diode 256.

The AC supply 252 may be used to modulate the LED or laser diode 256 with data or for other reasons. In one embodiment, the AC supply 252 is configured to operate at between 1 KHz and 200 MHz. Other embodiments of the invention may allow for frequencies up to 2.5 GHz and beyond.

When the test apparatus 200 shown in FIG. 2 is used for testing laser diodes and LEDs, the transmitter source may be configured to couple to a transmitter such as an LED or laser diode. This configuration may include an appropriate test fixture that allows for quick removal and replacement of transmitters in the test fixture. The LED or laser diode 256 may be optically coupled to a receiver 102. The optical coupling shown in FIG. 2 includes a path through a patch cord 258. This allows the optical signal produced by the LED or laser diode 256 to be transmitted to the receiver

The receiver 102 includes a photodiode 260 and a transimpedance amplifier 262. The photodiode 260 converts optical signals received from the patch cord 258 to a small electrical current through the photodiode 260. The transimpedance amplifier 262 converts the small current through the photodiode 260 into a higher power differential electrical signal that is output as a differential signal on a positive output 264 and a negative output 268. The differential signal includes a positive differential signal 270 and a negative differential signal 272. The positive differential signal is fed to a first impedance matching network 274. The negative differential signal is sent to a second impedance matching network 276.

The first and second impedance matching networks 274, 276 may be configured to match the line characteristics from the output of the transimpedance amplifier 262. In one embodiment, the impedance matching networks 274, 276 are fabricated on a printed circuit board that includes various paths for receiving different values of components such as capacitors, resistors and inductors. Thus a printed circuit board can be customized for a particular test by stuffing the board with appropriately chosen components. Embodiments of the invention contemplate the use of several different kinds of matching networks. For example and not by way of limitation, a matching network may include specially designed printed circuit board traces that have a particular capacitance, inductance and/or resistance. The matching networks may include fixed components such that the matching networks are fixed for a particular application or use. The matching networks may include switched components such that the matching network may be used for a plurality of different applications with minimal reconfiguration. The matching networks may comprise a variable filter for even further flexibility in designing tests apparatus. In one example the matching networks may include a digital signal processor (DSP) that functions as a filter. In some embodiments the first and second impedance matching networks 274, 276 are designed with similar or complementary printed circuit board layouts and components. This helps to ensure that the positive differential signal 270 and the negative differential signal 272 remain in phase with respect to each other.

The positive differential signal passes through the first impedance matching network 274 to a first filter 278. The first filter 278 separates AC and DC signals from the positive differential signal 270. The DC portions of the positive differential signal 270 are fed to a DC output 114. The negative differential signal 272 follows a similar path through the second impedance matching network 276 to a second filter 280 where the DC portion of the negative differential signal 272 is output at a DC output 116. The first and second filters 278, 280 may be in one example bias tees.

The AC output from the first and second filters 278, 280 is fed into an amplifier 282. The amplifier 282 in one embodiment is a high frequency amplifier with a wide bandwidth, low noise and other desirable characteristics. One example of an amplifier that may be used is the AD8129 available from Analog Devices. This particular amplifier functions at frequencies up to 250 MHz. The amplifier 282 is the differential amplifier that compares the positive AC signal and the negative AC signal and produces a difference of the two AC signals. This difference of the two AC signal is fed to a true RMS converter 284.

The true RMS converter 284 converts the difference of the AC signals to a function of the RMS value of the difference of the AC signals. In one embodiment the true RMS converter 284 maybe part number AD8361 from analog devices. This particular true RMS converter outputs a signal that is generally 7.5 times the value of the RMS value of the difference of the AC signals. The function of the RMS value of the difference of the AC signals may vary slightly from the 7.5 value depending on the configuration of the true RMS converter 284. Alternate functions maybe readily obtained from the data sheet for this device which is available from analog devices on their website. As mentioned previously, the present embodiment shown is designed for operation between 1 KHz and 200 MHz. However, other embodiments may be designed to function up to 2.5 gigahertz and beyond. When embodiments are designed for frequencies above 200 MHz, typically the embodiments will be designed as a band pass filter for a range of frequencies so as to obtain the best results from of the true RMS converter 284. For example, it may be desirable to bias the true RMS converter 284 such that lower frequencies are less usable when the circuit is designed for higher frequencies. When constructing circuits for use above 200 MHz, an alternate amplifier 282 may be used that has a bandwidth suitable for use above 200 MHz.

In constructing the test apparatus 200 it is desirable to construct the apparatus using a printed circuit board layout for certain portions of the circuits. It may also be desirable to ensure that traces on the printed circuit board are matched for positive and negative signal paths. If the traces are not matched for positive and negative signal paths, positive and negative signals may vary slightly in their phase from each other resulting an erroneous readings from the RMS converter 284.

The apparatuses described herein should be calibrated when used as test equipment. However, using the components described herein as test apparatuses has been shown to be very accurate. Thus an automated calibration may be used where calibration equipment used in the automated calibration is calibrated so as to verify the accuracy of the automated calibration. In one embodiment, a buffer circuit connected to the output of the true RMS converter 284 may be used to compensate for a circuit that is out of calibration. The buffer circuit may be an amplifier circuit this about unity (i.e. a gain of 1), but adjustable slightly up or down to compensate for any inaccuracies.

The terms high and low frequency, as described herein should be considered relative terms rather than applying to a specific standard of frequencies. Thus high-frequency as used herein is used to describe communications that use relatively high modulation rates as opposed to a specific range of frequencies as is used in some areas of the electronic and communication arts.

The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope. 

1. A test apparatus comprising: a transmitter source were in the transmitter source comprises: an AC supply; and a DC supply; wherein the transmitter source is adapted to couple to a transmitter; an optical receiver adapted to receive optical signals from a transmitter coupled to the transmitter source, wherein the optical receiver comprises an amplifier with a differential output; a first matching network coupled to a high output of the differential output; a second matching network coupled to a low output of the differential output; a first filter coupled to the first matching network, the first filter being adapted to separate AC and DC signals; a second filter coupled to the second matching network, the second filter being adapted to separate AC and DC signals; an RMS conversion circuit coupled to the first and second filters such that the RMS conversion circuit receives AC signals from the first and second filters, wherein the RMS conversion circuit converts the AC signals to a DC function of the RMS value of the AC signals; and a data acquisition system coupled to the RMS conversion circuit to receive the function of the RMS value of the AC signals further wherein the data acquisition system is coupled to the first and second filters to receive DC signals from the first and second filters.
 2. The test apparatus of claim 1, wherein the optical receiver comprises a photodiode and a transimpedance amplifier.
 3. The test apparatus of claim 1, wherein the AC supply is configured to operate at about between 1 KHz and 200 MHz.
 4. The test apparatus of claim 1, wherein the first matching network and second matching network are matched with similar or complementary printed circuit board layouts.
 5. The test apparatus of claim 1, wherein the first matching network and second matching network include fixed components.
 6. The test apparatus of claim 1, wherein the first matching network and second matching network include a digital signal processor.
 7. The test apparatus of claim 1, wherein the first matching network and second matching network include switched components.
 8. The test apparatus of claim 1, wherein the first matching network and second matching network include traces designed for at least one of a particular capacitance, inductance and resistance.
 9. The test apparatus of claim 1, wherein the first filter and second filter are bias tees.
 10. The test apparatus of claim 1, wherein the RMS conversion circuit is included on a printed circuit board, the printed circuit board adapted to connect to the optical receiver.
 11. The test apparatus of claim 1, wherein the RMS conversion circuit is configured as a band pass filter designed for a range of frequencies.
 12. The test apparatus of claim 1, further comprising a buffer circuit coupled to the RMS conversion circuit for calibrating the test apparatus.
 13. A test apparatus comprising: a transmitter source were in the transmitter source comprises: an AC supply; and a DC supply; wherein the transmitter source is coupled to a transmitter; a fixture adapted to receive an optical receiver such that the optical receiver may receive optical signals from the transmitter coupled to the transmitter source, wherein the optical receiver comprises an amplifier with a differential output; a first matching network adapted to couple to a high output of the differential output; a second matching network adapted to couple to a low output of-the differential output; a first filter coupled to the first matching network, the first filter being adapted to separate AC and DC signals; a second filter coupled to the second matching network, the second filter being adapted to separate AC and DC signals; an RMS conversion circuit coupled to the first and second filters such that the RMS conversion circuit receives AC signals from the first and second filters, wherein the RMS conversion circuit converts the AC signals to a DC function of the RMS value of the AC signals; and a data acquisition system coupled to the RMS conversion circuit to receive the function of the RMS value of the AC signals further wherein the data acquisition system is coupled to the first and second filters to receive DC signals from the first and second filters.
 14. A method of testing optical components comprising: receiving an optical signal from an optical source, the optical signal including AC and DC components; converting the optical signal to an electrical signal; separating AC and DC components of the electrical signal; converting the AC component of the electrical signal to a function of the RMS value of the AC component of the electrical signal; and providing the function of the RMS value to a data acquisition system.
 15. The method of claim 14, further comprising performing calibration by comparing the function of the RMS value to a calibrated RMS value of the AC portion of the electrical signal.
 16. The method of claim 15, further comprising compensating for a difference that shows a mis-calibration between the function of the RMS value and the calibrated RMS value of the AC portion of the electrical signal. 